Transmitting photoelectric sensor array

ABSTRACT

A plurality of sensing arms are provided at intervals on a sensor case so as to project outwardly of the case. A light-projecting element and a photoreceptor element forming a pair are provided in the sensor case so as to correspond to respective ones of the sensing arms. Projected light from the light-projecting element advances along the longitudinal direction of the respective sensing arm and is reflected by a reflecting surface so as to be directed toward the neighboring sensing arm through a sensing area. The projected light is reflected twice by reflecting surfaces on the neighboring sensing arm so as to return to the original sensing arm through the sensing area again. The light is reflected by the reflecting surface of this sensing arm, advances along the longitudinal direction of this sensing arm again and is received by the photoreceptor element.

TECHNICAL FIELD

This invention relates to a multiple transmission-type photoelectricsensor for sensing a plurality of objects at one time, a singletransmission-type photoelectric sensor capable of being appliedparticularly to a multiple transmission-type photoelectric sensor, and aphotoelectric sensing method.

BACKGROUND ART

Available as one example of a multiple transmission-type photoelectricsensor is a wafer sensor used in a semiconductor-wafer manufacturingprocess to check semiconductor wafers to determine whether or not theyare present, monitor the wafers and verify the number thereof on alot-by-lot basis.

Sensors described in the specifications of Japanese Patent PublicationNo. 6-11070 and Japanese Utility Model Application Laid-Open No. 5-66987are examples of wafer sensors. The wafer sensors described in thisliterature include a light-projecting element and a photoreceptorelement forming a pair and disposed so as to oppose each other. Amultiplicity of these light-projecting and photoreceptor elements areinserted between wafers held in a wafer cassette at regular intervals insuch a manner that the wafers will be sandwiched by the pairs oflight-projecting and photoreceptor elements. A pair of thelight-projecting and photoreceptor elements sandwiching a waferconstructs a transmission-type photoelectric sensor. Light projectedfrom the light-projecting element is blocked if a wafer is present butis received by the corresponding photoreceptor element in the absence ofa wafer.

Since a semiconductor wafer is opaque, its absence or presence can besensed on the basis of whether or not the projected light is blocked, asdescribed above. In recent years, transparent or semi-transparent wafersthat rely on quartz glass, sapphire glass, liquid-crystal glass andsilicon-carbide glass have come to be used for a variety ofapplications. These transparent or semi-transparent wafers cannot besensed by, or are difficult to sense by, the above-mentioned wafersensor. The reason for this is that a transparent wafer transmits mostof the projected light from the light-projecting element so that theprojected light reaches the photoreceptor element with littleattenuation. The difference in amount of light received by thephotoreceptor element when a transparent wafer is and is not present isvery small and is difficult to identify. In addition, extraneous lightand a change in the characteristics of the light-projecting andphotoreceptor elements due to temperature are not negligible.

A wafer sensor that is applicable to both transparent and opaque wafersis illustrated in the specification of Japanese Patent ApplicationLaid-Open No. 6-77307. This wafer sensor includes a first element havinga light-emitting surface and a photoreceptor surface, and a secondelement having a photoreceptor surface, the elements being disposed soas to oppose each other. A wafer is inserted between the first andsecond elements. For a wafer that is opaque, light projected from thefirst element is blocked by the wafer if the wafer is present. If thewafer is absent, the projected light is received by the second element.In the case of a transparent wafer, light projected from the firstelement and reflected by the wafer (if the wafer is present) is receivedby the photoreceptor surface of the first element. If the wafer isabsent, light does not impinge upon the photoreceptor surface of thefirst element. This is premised on the fact that whether the wafer is atransparent wafer or an opaque wafer is known beforehand. Thephotoreception signal of the first element and the photoreception signalof the second element are switched, depending upon the type of wafer,before being applied to a discriminating circuit.

This wafer sensor requires the first element having the light-emittingand photoreceptor surfaces. In the case of a transparent wafer, thesensor receives the light reflected from the wafer and therefore isreadily influenced by the surface of the wafer. The wafer type, i.e.,transparent or opaque, must be known in advance.

In the wafer sensors of all of the above-mentioned types, alight-projecting (light-emitting) element and a photoreceptor elementmust be inserted in the gaps between wafers. There has been a tendencyin recent years for the gaps between the multiplicity of wafers held inthe wafer cassette to be made smaller. There is a limit upon the extentto which the thickness of the light-projecting and photoreceptorelements and the thickness of the members for holding these elements canbe reduced.

There are occasions where the wafers in a semiconductor process becomecharged with static electricity. If this static electricity dischargesthrough electrically conductive portions of the light-projecting andphotoreceptor elements inserted between the wafers, there is the dangerthat this may lead to erroneous detection and destruction of thelight-projecting and photoreceptor elements.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a transmission-typephotoelectric sensor, multiple transmission-type photoelectric sensorand photoelectric sensing method capable of sensing both opaque andtransparent bodies (inclusive of semi-transparent bodies).

Another object of the present invention is to provide a structurewhereby a sensing portion inserted between objects to be sensed can bemade as thin as possible.

A further object of the present invention is to provide a structurewhereby the influence of static electricity that has charged an objectto be sensed can be made as small as possible.

Still another object of the present invention is to provide a structurewhereby the number of light-projecting and photoreceptor elements can bemade as small as possible.

A multiple transmission-type photoelectric sensor according to thepresent invention is defined as follows when expressed all-inclusively:Specifically, a multiple transmission-type photoelectric sensoraccording to the present invention has a plurality of sensing armsprovided in spaced-apart relation on a sensor case so as to extendoutwardly of the case, and a plurality of light-projecting elements anda plurality of photoreceptor elements provided inside the sensor case,one light-projecting element, one photoreceptor element or one pair ofthe light-projecting and photoreceptor elements corresponding to eachsensing arm, a distal end of each sensing arm being provided with atleast one of a first deflecting member for directing projected lightfrom the corresponding light-projecting element toward a neighboringsensing arm and a second deflecting member for directing projected lightfrom the neighboring sensing arm toward the corresponding photoreceptorelement.

In one embodiment, one pair of the light-projecting and photoreceptorelements corresponds to each sensing arm, and each sensing arm isprovided with the first deflecting member and the second deflectingmember. In another embodiment, one light-projecting element or onephotoreceptor element corresponds to each sensing arm, the distal end ofthe sensing arm that corresponds to the light-projecting element isprovided with the first deflecting member, and the distal end of thesensing arm that corresponds to the photoreceptor element is providedwith the second deflecting member.

In any case, the space between two neighboring sensing arms is a sensingarea, projected light from the light-projecting element of one sensingarm reaches the neighboring other sensing arm by traversing the sensingarea at least one time (two or more times depending upon the mode) andis received by the photoreceptor element of the one sensing arm or ofthe other sensing arm. By subjecting the photoreception signal of thephotoreceptor element to level discrimination, at least the absence orpresence of an object in the sensing area is determined.

In accordance with the present invention, a plurality of sensing armsare provided. As a result, a plurality of sensing areas are establishedand sensing operations can be performed simultaneously in theseplurality of sensing areas. The distal end of each sensing arm need onlybe provided with a deflecting member (e.g., a reflecting surface, aprism, etc.); provision of a light-projecting element and photoreceptorelement is not necessary. Accordingly, it is possible to reduce thethickness of the sensing arm so that the sensing arm can be applied to anarrow sensing area. In a case where the photoelectric sensor accordingto the present invention is used in sensing a wafer or the like, thefact that the distance between the wafer and the light-projecting andphotoreceptor elements is great (a long distance can be set) means thateven if the wafer becomes charged with static electricity, it ispossible to prevent the light-projecting and photoreceptor elements andthe sensing circuit from being adversely affected by this staticelectricity.

In a preferred embodiment, each sensing arm is provided with a thirddeflecting member for returning the projected light from the neighboringsensing arm to this neighboring sensing arm.

The projected light traverses the space (the sensing area) betweenneighboring sensing arms at least two times. Even if an object to besensed is transparent or semi-transparent, the projected light passesthrough the object at least twice, as a result of which the amount ofattenuation increases to make possible reliable detection of atransparent or semi-transparent object.

In another preferred embodiment, the first deflecting member directs theoptical path of the projected light obliquely with respect to adirection in which the sensing arms are arrayed, and the seconddeflecting member directs the light, which has advanced obliquely withrespect to the direction in which the sensing arms are arrayed, towardthe photoreceptor element.

The projected light impinges upon the surface of the object within thesensing area obliquely and passes through the object obliquely. Sinceloss equivalent to the amount of reflection of the light that impingesobliquely upon the surface of the object is great, it is possible tosense the object reliably regardless of whether it is transparent orsemi-transparent.

When the projected light passes through the object obliquely, the opticaxis is displaced. It is preferred that the front side of thephotoreceptor element be provided with a slit which limits the incidentlight in such a manner that the displacement of the optic axis can besensed more noticeably. The amount of light that impinges upon thephotoreceptor element varies greatly depending upon whether or not anobject is present (even if the object is a transparent object) and thetype of object, as a result of which more certain detection becomespossible.

The displacement of the optic axis can be sensed by a position sensingdevice and the absence or presence of an object and the type thereof canbe determined based upon an output representing the position sensed bythe position sensing device.

If the output representing the sensed position and a photoreceptionoutput are obtained from the position sensing device, these outputs areeach discriminated by a predetermined threshold value and the results ofdiscrimination are subjected to a logic operation, it is possible tosense an object much more reliably (as well as the type of object whennecessary).

In yet another preferred embodiment, the first deflecting member splitsthe projected light from the light-projecting element into two portionsand directs these two portions toward the neighboring sensing arms onboth sides.

Since the projected light from one light-projecting element is splitinto two portions and propagates into two sensing areas, the number oflight-projecting and photoreceptor elements can be reduced.

In order that objects to be sensed in a plurality of sensing areas maybe sensed in time-shared fashion, there are provided drive means fordriving a plurality of light-projecting elements sequentially atpredetermined time intervals, and means for fetching, in sync withdriving of the light-projecting element, a photoreception signal of aprescribed photoreceptor element which receives projected light from alight-projecting element that is driven. There is further provideddecision means having at least one threshold value for discriminating anoutput signal of a photoreceptor element based upon this thresholdvalue, thereby outputting a detection signal indicative of an object tobe sensed.

There are a variety of threshold values. One is for distinguishingbetween absence of a transparent body and presence of a transparentbody. A second is for distinguishing between absence of asemi-transparent body and presence of a semi-transparent body. A thirdis for distinguishing between a transparent body and a semi-transparentbody. A fourth is for distinguishing between a transparent body and anopaque body. A fifth is for distinguishing between a semi-transparentbody and an opaque body. A sixth is for distinguishing between absenceor presence of an object. These threshold values may be combinedappropriately.

In one embodiment, the sensing arms are provided with at least one of afirst light guide for guiding projected light from a light-projectingelement to the first deflecting member and a second light guide forguiding light from the second deflecting member to a photoreceptorelement. Light can be guided reliably in the sensing arm and the effectsof extraneous light can be reduced.

Preferably, the case is provided with at least one of a shield membercovering the light-projecting element, a shield member covering thephotoreceptor element, a shield member covering the space between thesensing arms, a shield member covering the entirety of the plurality ofphotoreceptor elements, and a shield member covering a circuit board onwhich the sensing circuit is provided. This furnishes a greatly enhancedstatic-electricity countermeasure.

A first type of single transmission-type photoelectric sensor (which canalso be used in multiple-type configuration as a matter of course)according to the present invention has a light-projecting element and aphotoreceptor element for receiving light that has traversed a sensingarea upon being emitted by the light-projecting element, characterizedby having: a first deflecting member for bending, approximately at rightangles, projected light from the light-projecting element to direct theprojected light toward the sensing area; a second deflecting member forbending, approximately at right angles, light from the sensing area todirect the projected light toward the photoreceptor element; and a thirddeflecting member for directing the light, which has been introduced bythe first deflecting member and has traversed the sensing area, towardthe second deflecting member via the sensing area again.

Preferably, the third deflecting member includes two deflecting membersfor bending incident light approximately at right angles and directslight, which is approximately parallel to the light introduced to thesensing area by the first deflecting member, toward the seconddeflecting member.

Projected light from the light-projecting element is directed from thefirst deflecting member to the third deflecting member through thesensing area, advances from the third deflecting member to the seconddeflecting member through the sensing area again and arrives at thephotoreceptor element.

The arrangement is such that the projected light traverses the sensingarea at two or more times. As a result, if a transparent orsemi-transparent body is present in the sensing area, the projectedlight passes through the transparent or semi-transparent body at leasttwo times. Since the amount of attenuation of the projected light isincreased, even a transparent body (or semi-transparent body) can besensed with certainty. Since the arrangement is such that the projectedlight arrives at the photoreceptor element upon being deflectedapproximately at right angles by the first deflecting member anddeflected approximately at right angles by the second deflecting member,the projected light does reach the photoreceptor element directly. Thismakes it possible to prevent erroneous detection due to reflection orthe like.

The first type of transmission-type photoelectric sensor can be used ina multiple-type configuration as well. In such case it is preferred thatthe photoelectric sensor include at least two, namely first and second,sensing arms disposed so as to oppose each other across a sensing areadefined therebetween. The first sensing arm has, at a distal endthereof, a first deflecting member for deflecting projected light from alight-projecting element, which projected light is introduced from abase end of the first sensing arm and advances along the longitudinaldirection of the arm, in a direction approximately perpendicular to thelongitudinal direction of the arm, thereby directing the light towardthe sensing area; and, at the distal end thereof, a second deflectingmember for deflecting light from the sensing area in a directionapproximately at right angles, causing the light to advance along thelongitudinal direction of the arm and introducing the light to thephotoreceptor element. The second sensing arm has a third deflectingmember for causing light, which has advanced from the first deflectingmember via the sensing area, toward the second deflecting member via thesensing area again.

A photoelectric sensing method according to the present invention can beexpressed as follows: Specifically, a photoelectric sensing methodaccording to the present invention includes causing light from alight-projecting element to advance to a sensing area upon beingdeflected approximately at right angles, directing the light, which hastraversed the sensing area, toward the sensing area again to therebycause the light to traverse the sensing area at least two times,introducing the light, which has traversed the sensing area at least twotimes, to a photoreceptor element upon deflecting the lightapproximately at right angles, and sensing an object, which is presentin the sensing area, based upon an output signal from the photoreceptorelement.

A second type of transmission-type photoelectric sensor has at leastfirst and second sensing arms disposed so as to oppose each other acrossa sensing area defined therebetween. The first sensing arm has, at adistal end thereof, a first deflecting member for deflecting projectedlight from a light-projecting element, which projected light isintroduced from a base end of the first sensing arm and advances alongthe longitudinal direction of the arm, in a direction that isapproximately perpendicular to the longitudinal direction of the arm andoblique with respect to a direction in which the first and secondsensing arms are arrayed, thereby directing the light toward the sensingarea. The second sensing arm has, at a distal end thereof, a seconddeflecting member for deflecting light, which has advanced from thefirst deflecting member of the first sensing arm obliquely through thesensing area, in an approximately perpendicular direction, causing thelight to advance along the longitudinal direction of the arm andintroducing the light to a photoreceptor element.

The arrangement is such that the projected light crosses the sensingarea obliquely. The projected light impinges obliquely upon the surfaceof an object to be sensed. Since loss due to reflection of the obliquelyincident light is large, the fact that the difference in amount of lightincident upon the photoreceptor element when an object to be sensed isand is not present is large makes it possible to sense the object withcertainty, even if the object to be sensed is transparent (orsemi-transparent).

In a preferred embodiment, both the first sensing arm and the secondsensing arm are provided with the first and second deflecting members.

Preferably, the front side of the photoreceptor element is provided witha slit which limits the incident light. The photoreceptor element may bea position sensing device.

A third type of transmission-type photoelectric sensor includes a firstsensing arm, and second and third sensing arms disposed on respectiveones of both sides of the first sensing arm across sensing areas definedtherebetween. The first sensing arm has, at a distal end thereof, afirst splitting and deflecting member for splitting, into two portions,projected light from a light-projecting element, which projected lightis introduced from a base end of the first sensing arm and advancesalong the longitudinal direction of the arm, deflecting these splitportions of light approximately at right angles, and directing thesesplit portions of light toward the sensing areas on both sides. Thesecond and third sensing arms respectively have, at respective distalends thereof, second and third deflecting members, respectively, fordeflecting light, which has advanced from the first splitting anddeflecting member through the sensing area, approximately at rightangles, causing the light to advance along the longitudinal direction ofthe arm and introducing the light to photoreceptor elements.

Projected light from one light-projecting element is split into twoportions by the first splitting and deflecting member and advances intothe two sensing areas. The two beams of light that have traversed thesesensing areas are deflected by respective ones of the second and thirddeflecting members and are received by respective ones of separatephotoreceptor elements. Sensing in regard to objects in two sensingareas is possible by a single light-projecting element, thereby reducingthe number of elements. In a case where this photoelectric sensor isapplied to a multiple-type configuration, the numbers oflight-projecting elements and photoreceptor elements can be reducedgreatly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the manner in which a wafer sensoris used to sense wafers in a wafer cassette;

FIG. 2 is a perspective view showing a part of the wafer sensor inenlarged form;

FIG. 3 illustrates the sensing principle of a wafer sensor according toa first embodiment;

FIG. 4 is a perspective view of a sensing arm according to the firstembodiment as seen from the front;

FIG. 5 is a perspective view of a sensing arm according to the firstembodiment as seen from the back;

FIG. 6 is a back view of the same sensing arm;

FIG. 7 is a sectional view of FIG. 4 taken along line VII—VII;

FIG. 8 is a sectional view of FIG. 4 taken along line VIII—VIII;

FIG. 9 is a block diagram showing a sensing circuit;

FIG. 10 is a time chart illustrating the operation of the sensingcircuit;

FIG. 11 is a perspective view of a sensing arm according to a firstmodification as seen from the front;

FIG. 12 is a sectional view of FIG. 11 taken along line XII—XII;

FIG. 13 is an exploded perspective view of the sensing arm according tothe first modification;

FIG. 14 is a perspective view of a sensing arm according to a secondmodification, a portion of which is cut off;

FIG. 15 is a sectional view of FIG. 14 taken along line XV—XV;

FIG. 16 illustrates another sensing principle;

FIG. 17 illustrates yet another sensing principle;

FIG. 18 is a perspective view showing several sensing arms illustratinga second embodiment;

FIGS. 19a and 19 b illustrate the sensing principle of a wafer sensoraccording to a second embodiment;

FIG. 20 is a block diagram showing an example of a sensing circuitaccording to the second embodiment;

FIG. 21 is a perspective view of sensing arms illustrating a first modein a third embodiment;

FIG. 22 is a longitudinal sectional view of part of each of the sensingarms shown in FIG. 21;

FIG. 23 is a perspective view of the overall wafer sensor;

FIG. 24 illustrates the sensing principle of the first mode;

FIG. 25 is a perspective view of sensing arms illustrating a second modein the third embodiment;

FIG. 26 illustrates the sensing principle of the second mode;

FIG. 27 is a block diagram showing an example of a sensing circuitaccording to the third embodiment;

FIG. 28 is a time chart illustrating the operation of a sensing circuitaccording to the third embodiment;

FIG. 29 is a front view of a wafer sensor illustrating a fourthembodiment;

FIG. 30 is a front view in which the interior of the wafer sensor isexposed by removing a cover;

FIG. 31 is an enlarged sectional view in which the interior of the wafersensor is shown by repeatedly breaking it away;

FIG. 32 is an exploded perspective view showing a shield plate and ashield piece; and

FIG. 33 is a transverse sectional view of the wafer sensor.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 1 illustrates the manner in which a multiplicity of wafers 9 areheld in a wafer cassette (or wafer carrier) 8 in a process formanufacturing a semiconductor device. The wafer cassette 8 has two sidewalls formed so as to approach each other as the bottom of the cassetteis approached, the bottom having an opening. The inner surface of eachof the two side walls of wafer cassette 8 is formed to have amultiplicity of vertically extending retaining grooves (not shown) atprescribed intervals. Each wafer 9 is held in the retaining groove so asto be free to move up and down and free to rotate. The spacing betweenthe wafers 9 is decided by the spacing between the retaining grooves. Analignment roller 7 rotates the wafers 9 inside the cassette 8 to alignthe wafers (adjust their attitude) in an alignment part of themanufacturing process. For example, the wafers are aligned in such amanner that orientation flats on all of the wafers 9 will be situated atthe bottom.

As shown in FIG. 2, a wafer sensor 10 includes a sensor case 11 and amultiplicity of sensing arms 12 arrayed on the top side of the case atprescribed intervals. The sensing arms 12 project upwardly from the topside of the sensor case 11. The intervals of the sensing arms 12 and theintervals of the wafers 9 held in the wafer cassette 8 are equal.

The wafer sensor 10 is provided on an elevator base 6 and is raised byraising the elevator base 6 so that the sensing arms 12, with theexception of those at both ends, enter the spaces between the wafers 9on one side of the alignment roller 7. In other words, each wafer 9 isinterposed between two neighboring sensing arms 12.

As will become clear from a detailed description given later, eachsensing arm 12 constructs part of a transmission-type photoelectricsensor. The wafer sensor 10 senses whether wafers are contained in thewafer cassette 9, the number thereof, etc. The wafer sensor 10 issubsequently lowered and the wafer cassette 9 is sent to the nextprocess.

FIG. 3 illustrates the sensing principle of the wafer sensor 10 (thetransmission-type photoelectric sensor or a multiple transmission-typephotoelectric sensor). Though the above-mentioned wafer can be cited asone example of an object to be sensed, various other objects can beadopted irrespective of whether they are opaque, transparent orsemi-transparent. For the sake of convenience in terms of explanation,an object OB is assumed to be transparent (or semi-transparent).

A light-projecting element 21 and a photoreceptor element 22 aredisposed side by side with a space between them. The optic axes of thelight-projecting element 21 and photoreceptor element 22 are parallel(or approximately parallel). Four reflecting surfaces (e.g., mirrorsurfaces or mirrors) are disposed as follows in relation to the objectOB in order to form optical paths L1, L2, L3, L4 and L5 along whichprojected light emitted by the light-projecting element 21 arrives atthe photoreceptor element 22:

The light-projecting element 21 includes a projecting lens (not shown)(or a projecting lens is provided in front of the light-projectingelement 21) so that projected light that has been collimated will beemitted from the light-projecting element 21. The projected lightadvances along the optic axis of the light-projecting element 21(optical path L1) and is reflected by a reflecting surface 13A so as tobe directed along the optical path L2. The optical path L2 isperpendicular (or approximately perpendicular) to a plane (referred toas a “reference plane” below) containing the optic axis (optical pathL1) of light-projecting element 21 and the optic axis (optical path L5)of photoreceptor element 22. The projected light which advances alongthe optical path L2 impinges upon the object OB perpendicularly (orapproximately perpendicularly).

The light that has impinged upon the object OB is attenuated slightly inthe process of passing through the object OB. The light that has passedthrough the object OB is reflected again by a reflecting surface 14AA sothat the optical path thereof is bent at right angles (or approximatelyright angles). The optical path L3, which is perpendicular (orapproximately perpendicular) to the optical path L2 lies parallel (orapproximately parallel) to the reference plane. The light which advancesalong the optical path L3 is reflected further by a reflecting surface14BB so as to advance along the optical path L4. The optical path L4 isperpendicular (or substantially perpendicular) to the optical path L3and parallel (or substantially parallel) to the optical path L2.

The light which advances along the optical path L4 impinges upon theobject OB again. The light is attenuated slightly in the process ofpassing through the object OB. The light that has passed through theobject OB is reflected again by a reflecting surface 13B so that thelight is directed toward the photoreceptor element 22 along the opticalpath L5, condensed by a photoreceptor lens (not shown) and caused toimpinge upon the photoreceptor element 22.

The result is that the projected light passes through the object OBtwice. The optical paths L2 and L4 are the optical paths of light thattraverses a sensing area. The plane defined by the optical paths L2 andL4 is perpendicular (or substantially perpendicular) to the referenceplane. In a case where the object OB is planar, it is preferred that theobject OB be placed parallel (or approximately parallel) to thereference plane.

Even if the object OB is a transparent body, its transmittance is not100% and light is attenuated upon passing through the object OB. Sincethe projected light passes through the object OB twice, the attenuationof the light is increased as compared with transmission through theobject once.

By way of example, assume that the transmittance of a transparent waferis 92%. Assume that the amount of light received by the photoreceptorelement 22 (the level of the photoreception signal) when an object to besensed (the transparent wafer) is not present is 100. Since theprojected light passes through the transparent wafer twice, the amountof light received by the photoreceptor element 22 is 100×0.92×0.92=84.6.By setting a threshold value in the vicinity of 90 and comparing theamount of light received by the photoreceptor element 22 with thisthreshold value, whether or not the transparent wafer is present can besensed. When an opaque wafer is present as the object OB, the projectedlight led along the optical path L2 is interrupted by the opaque waferin the sensing area and the amount of light received by thephotoreceptor element 22 becomes approximately zero. If two values,e.g., 60 and 90, are set as threshold values, it is possible to performsensing that distinguishes between presence of a transparent wafer andpresence of an opaque wafer. It is judged that an opaque wafer ispresent if the amount of received light is less than 60, that atransparent wafer is present if the amount of received light is 60 orgreater but less than 90, and that a wafer is absent if the amount ofreceived light is 90 or greater. Thus, not only the absence or presenceof a wafer but also the type of wafer can be determined. In addition totransparent and opaque wafers, it is possible to identify and sensesemi-transparent wafers as well in the same way. It will suffice topreviously determine the amount of light received by the photoreceptorelement when light has passed through a semi-transparent wafer twice,and set three different threshold values.

It is preferred that the two parallel optical paths L2 and L4 be formedat spaced-apart positions in the sensing area. The reason for this is asfollows: The surface of an opaque wafer generally is a mirror surface.If the two optical paths L2 and L4 were close together, therefore, theprojected light from the light-projecting element 21 would be positivelyreflected by the surface of the wafer and reach the photoreceptorelement 22 via the reflecting surface 13B when the projected lightimpinges upon the wafer upon being reflected by the reflecting surface13A and advancing along the optical path L2. In this case the amount oflight received by the photoreceptor element 22 would exceed thethreshold value 90 and there is the danger that the wafer would besensed erroneously as being absent despite the fact that it is present.

Setting the optical paths L2 and L4 perpendicular (or approximatelyperpendicular) to the surface of the wafer also is for the purpose ofassuring that light reflected by the surface of the wafer will notimpinge upon the photoreceptor element 22. The reason for this is thatif the projected light impinges upon the surface of the waferperpendicularly, the reflected light, even if it returns to thereflecting surface 13A or light-projecting element 21, will not bedirected toward the reflecting surface 13B or photoreceptor element 22.

Further, the reason for forming the optical paths L2 and L4perpendicular (or approximately perpendicular) to the reference plane inthe sensing area is to assure that reflected light, which is the resultof the projected light being reflected by the surface of the wafer, willnot return to the photoreceptor element 22 directly.

In accordance with the sensing principle described above, the sensingarm 12 is formed to have the reflecting surfaces 13A, 13B and reflectingsurfaces 14A, 14B (which correspond to reflecting surfaces 14AA, 14BB)for reflecting light from the sensing arm neighboring on one side. Thereflecting surfaces 14AA and 14BB are formed as the reflecting surfaces14A, 14B on the sensing arm neighboring on the other side. The sensingarm 12 penetrates the space between wafers 9 and two neighboring sensingarms 12 sandwich a wafer 9 between them. The optical paths L2, L4 alongwhich light passes through the wafer 9 twice are formed between twosensing arms 12. Since the spacing between the wafers 9 is small, thesensing arm 12 has a thin, plate-like shape so that it can penetratethis narrow space.

The details of construction of the sensing arm 12 will now be describedwith reference to FIGS. 4 to 8.

The sensing arm 12 is constituted by an arm frame 15 and a mountingpiece 16. These are formed integrally from black, opaque resin (aninsulating resin). The base portion of the arm frame 15 indicated byline A—A and the mounting piece 16 are inserted into a recess or hole inthe sensor case 11, whereby the sensing arm 12 is mounted with its armframe 15 projecting outwardly from the case 11.

The mounting piece 16 is formed to have two element accommodatingrecesses 17, 18. The light-projecting element 21 comprises alight-emitting diode (LED), for example, and a projecting lens providedin front of light-emitting surface of the light-emitting diode. Thelight-projecting element 21 is press-fitted into the recess 17 and heldin the recess 17. The photoreceptor element 22, which is a photodiode,is press-fitted into and held in the accommodating recess 18 in a statein which it is housed within a shield case 23 consisting of anelectrically conductive material. The shield case 23 covers thephotoreceptor element 22 except for its photoreceptor surface and leads.The front sides of the accommodating recesses 17, 18 (the sides facingthe arm frame 15) are provided with light-projecting and photoreceptorwindows, respectively. The light-projecting surface of thelight-projecting element 21 and the photoreceptor surface of thephotoreceptor element 22 accommodated within the recesses 17, 18,respectively, are faced toward the light-projecting window andphotoreceptor window, respectively.

The arm frame 15 is constituted by two members 15A, 15B extending inparallel, and a member 15C connecting the distal ends of the members 15Aand 15B. Light from the light-projecting element 21, which isaccommodated within the recess 17, emitted via the light-projectingwindow advances outwardly along the inner side of the member 15A (thisis optical path L1). Further, light which advances inwardly along theinner side of the member 15B (optical path L5) is received by thephotoreceptor element 22, which is inside the recess 18, from thephotoreceptor window of the recess.

FIGS. 4 and 5 are perspective views of the sensing arm 12 as seen fromopposite sides thereof. For reasons of expediency in terms ofexplanation, the side seen in FIG. 4 shall be referred to as the frontside and the side seen in FIG. 5 shall be referred to as the back side.

A reflecting member 13 is integrally formed on the inner side of theconnecting member 15C of the arm frame 15 of sensing arm 12 so as toprotrude from the connecting member. With reference particularly toFIGS. 4, 7 and 8, the reflecting member 13 is formed to have the tworeflecting surfaces 13A, 13B, one on its left side and one on its rightside, on the front side of the arm frame 15. The reflecting surfaces13A, 13B are oblique surfaces which face the mounting piece 16 at anangle of 45°. Projected light (optical path L1) from thelight-projecting element 21 is reflected by the reflecting surface 13Aand directed toward the sensing area (wafer 9) along the optical pathL2. Light impinging from the sensing area (wafer 9) along the opticalpath L4 is reflected by the reflecting surface 13B and directed alongthe optical path L5 so as to reach the photoreceptor element 22.

With reference particularly to FIGS. 5, 6, 7 and 8, the reflectingmember 13 is formed to have the two reflecting surfaces 14A, 14B, one onits left side and one on its right side, on the back side of the armframe 15. The reflecting surfaces 14A, 14B face each other and have aninclination of 45°. Light that has been reflected by the reflectingsurface 13A of the neighboring sensing arm passes through the sensingarea (wafer 9) and impinges upon the reflecting surface 14A along anoptical path L2A (which corresponds to the above-mentioned optical pathL2). This light is reflected by the reflecting surface 14A and impinge supon the reflecting surface 14B via an optical path L3AB (whichcorresponds to optical path L3). The incident light is further reflectedby the reflecting surface 14B and is directed toward the sensing area(wafer 9) along an optical path L4B (which corresponds to the opticalpath L4).

The reflecting surfaces 14A, 14B on the back side are formed on sidessubstantially opposite the reflecting surfaces 13A, 13B on the frontside. The reflecting surfaces 13A, 13B, 14A, 14B can be formed byaffixing mirrors to, or vapor-depositing a metal such as aluminum on,the reflecting member 13, which is formed from a black, opaque resin, atthe positions of these reflecting surfaces. The reflecting surfaces 13Aand 13B on the front side are connected by a member 13 a of somewhatnarrow width. The member 13 a may or may not be a mirror surface, but itis better if it is not made a mirror surface. As a result, even ifprojected light from the light-projecting element 21 and light from thesensing area directed toward the photoreceptor element 22 should possesssome spread, these light rays will be separated to make possible theelimination of stray light, which is a chief cause of malfunction.Similarly, it is better that a surface 14 a (which is perpendicular tothe front or back side of the arm frame 15) between the reflectingsurfaces 14A and 14B also not be made a reflecting surface and be leftas a black resin. Preferably, the surface 14 a should be worked toreduce reflection or coated with a light-absorbing material.

Thus, as set forth above, the sensor case 11 is provided projectinglywith the multiplicity of sensing arms 12, each sensing arm 12 isprovided with a pair of light-projecting and photoreceptor elements 21,22, the sensing arms 12 are caused to penetrate the spaces between themultiplicity of wafers 9 arrayed at fixed intervals, projected lightfrom the light-projecting element 21 is directed from the distal end ofthe sensing arm 12 to the neighboring sensing arm, and this light isreflected by the neighboring sensing arm so as to return to theabove-mentioned sensing arm and be received by the photoreceptor element22. By adopting such an arrangement, the presence of a multiplicity ofwafers can be sensed at one stroke.

Since the projected light passes through the wafer twice before beingreceived by the photoreceptor element, it is possible to sense whetheror not the wafers are present, regardless of whether the wafers aretransparent or opaque (or semi-transparent), and to determine the typethereof (transparent, opaque, semi-transparent).

Since the sensing arm 12 need only be provided with members (the armframe and reflecting surfaces) for guiding and reflecting light, thesensing arm can be made thin so that sensing arms can be applied to agroup of wafers of small spacing.

Since the distance between the distal end of the sensing arm 12 insertedbetween the wafers 9 and the light-projecting and photoreceptor elements21, 22 is comparatively long, the distance acts as an insulatingdistance. The structure is such that static electricity that hasaccumulated on the wafer 9 will not readily discharge through thelight-projecting and photoreceptor elements 21, 22 or their circuits.Since the photoreceptor element 22 is shielded by the shield case 23, itwill not readily be destroyed by discharge (the shield case 23 beinggrounded).

The surface of the portion (the arm frame 15) of the sensing arm 12projecting from the case 11 desirably is subjected to a fluorine coatingtreatment. Chemicals often are used in the manufacture and machining ofwafers. When a wafer sensor approaches the vicinity of a wafer to whichstrongly acidic chemicals have attached themselves, it is possible thatthese strongly acidic chemicals will attach themselves to the arm frame15. Corrosion due to these strongly acidic chemicals can be prevented bythe fluorine coating. Applying the fluorine coating hardens the surfaceof the arm frame 15. Even if a wafer or some other object should strikethe sensing arm 12, chipping or cracking of the arm can be prevented.

In the above-described embodiment, the arm frame 15 and mounting piece16 are integrally formed, but these can be made separate bodies. The armframe 15 may be formed as an integral part of the case 11 only themounting piece 16 may be mounted on the case 11. A plurality of mountingpieces can also be integrally formed.

The number of sensing arms 12 provided is one greater than the number ofwafers 9 that are to be sensed at one stroke. For example, the wafersensor 10 applied to the wafer cassette 8 capable of accommodating amaximum of 25 of the wafers 9 would be provided with 26 sensing arms 12.The light-projecting and photoreceptor elements 21, 22 would be providedon 25 of these sensing arms 12. Light-projecting and photoreceptorelements would not be required on the one sensing arm, situated on theoutermost side, whose only role is to reflect and return projected lightfrom the neighboring sensing arm.

A sensing circuit provided within the case 11 of the wafer sensor havingsuch a construction is illustrated in FIG. 9, and the operating timingthereof is shown in FIG. 10. It will be assumed that 25 pairs oflight-projecting and photoreceptor elements constitute respectivechannels from 1 to 25 (1CH to 25CH).

A CPU 30 controls the projection of light and the reception of light andexecutes decision processing based upon a photoreception output appliedthereto.

The CPU 30 provides a decoder 31 with a signal that sequentiallydesignates the light-projecting elements 21 of 25 channels at regulartime intervals. The decoder 31 decodes the designating signal to open agate that is provided in a projected-light switching circuit 33 andcorresponds to the designated light-projecting element among the 25light-projecting elements 21, and to drive a light-projection driver 32.As a result, the 25 light-projecting elements 21 are driven sequentiallyat regular time intervals and output projected light in sequentialfashion. It goes without saying that it is so arranged that the drivetimes of the 25 light-projecting elements 21 will not overlap.

The decoder 31 further opens a gate of corresponding photoreceptorelement 22, the gate being provided in a photoreception switchingcircuit 34. The gates are opened at a timing somewhat later than that atwhich the gates of the light-projecting elements 21 are opened. The gateof one photoreceptor element 22 is opened at regular time intervals, andonly a photoreception signal from the photoreceptor element 22 isapplied to a band-pass filter 35 through the photoreception switchingcircuit 34. The photoreception signal is applied to a peak-hold circuit39 via the filter 35, a preamplifier 36, a main amplifier 37 and a combfilter 38. The comb filter 38 cuts low-frequency noise components. Thepeak-hold circuit 39 is reset at the timing at which a photoreceptorgate is opened. The signal held in the circuit 39 is accepted by the CPU30 as a photoreception output before the gate of the next photoreceptionsignal opens.

The CPU 30 judges whether a wafer is absent or present by comparing thelevels of photoreception outputs successively accepted in this mannerwith a threshold value that has been set in advance. If threshold valueshaving two or three levels are set, as mentioned above, the CPU 30 candetermine the wafer type (opaque, semi-transparent or transparent) aswell. Further, when necessary, the CPU 30 (acting as a wafer counter)counts the number of wafers present. Furthermore, the CPU 30 (acting asa wafer address sensor) generates an output indicating at what positiona wafer is present (and the type of wafer).

Before wafer sensing processing is executed, light may be projected fromthe light-projecting element 21 in the absence of a wafer 9, this lightmay be received by the photoreceptor element 22, and a threshold valuemay be set or corrected, treating the photoreception output of thephotoreceptor element 22 at this time as 100%.

The results of these decisions made by the CPU 30, or the output signalsfrom the CPU 30, are output externally from an output circuit 40 via acable 41. On the basis of these output signals, a controller controls awafer treating apparatus, an apparatus for manufacturing semiconductordevices, etc. The case 11 may be provided with 25 indicator lamps which,by being lit, would indicate the absence or presence of the wafers on anindividual basis. Modification

A modification of the first embodiment will now be described.

FIGS. 11 to 13 illustrate a sensing arm 12A according to a firstmodification. The sensing arm 12A uses an optical fiber in order toguide light in the sensing arm 12A and employs a prism to deflect light.Light-projecting and photoreceptor elements are not shown.

The sensing arm 12A includes an arm base 55 formed as an integral partof the mounting piece 16, and a cover 56 of the arm base 55. The armbase 55 is formed to have a thickened distal end, which is formed tohave recesses 53 a, 53 b in which prisms 53A, 53B, respectively, areaccommodated. The inner surface of the arm base 55 and the inner surfaceof the cover 56 are formed to have grooves 55 a, 55 b, 56 a, 56 b withsemi-circular cross sections for accommodating optical fibers 52A, 52B,respectively.

The cover 56 is bonded to the base 55 with the triangular prisms 53A,53B accommodated in the recesses 53 a, 53 b of base 55 and the opticalfibers 52A, 52B accommodated in the grooves 55 a, 55 b of base 55. Theprisms 53A, 53B have oblique surfaces which act as the reflectingsurfaces 13A, 13B. Projected light from the light-projecting elemententers the prism 53A through the optical fiber 52A, is reflected by thereflecting surface 13A and is emitted into the sensing area outside.Light from the sensing area enters the prism 53B, is reflected by thereflecting surface 13B, enters the optical fiber 52B and arrives at thephotoreceptor element through the optical fiber 52B.

The distal end of the base 55 has an underside which accommodates prisms(one prism 54B only is shown in FIG. 12) acting as the reflectingsurfaces 14A, 14B.

The sensing arm 12A of the first modification has the followingadvantages: The arm frame 15 of the sensing arm 12 in the firstembodiment has its central portion hollowed out. The sensing arm 12A ofthe first modification, however, does not possess a hollow space andtherefore has a higher strength and is strongly resistant to externalforces. The sensing arm 12A will not be easily damaged even if itstrikes the wafer cassette. There is also no need for an aluminumvapor-deposition process for the purpose of forming the reflectingsurfaces. Since light passes through the optical fibers 52A, 52B andprisms 53A, 53B, it is difficult for extraneous light to enter. Thiseliminates a cause of malfunction.

Lenses can be disposed between the optical fibers 52A, 52B and prisms53A, 53B to collimate the projected light and condense light from thesensing area. Optical elements having a lens function can be used as theprisms.

FIGS. 14 and 15 illustrate a second modification of the sensing arm.

Here a sensing arm 12B includes an arm frame 65 formed as an integralpart of the mounting piece 16. The arm frame 65 has a shape obtained byadding one additional member to the central portion of the arm frame 15of the first embodiment.

Optical guides 62A, 62B are provided between the outwardly extendingmembers of the arm frame 65 and the central member thereof. The opticalguides 62A, 62B have distal ends formed to include the obliquereflecting surfaces 13A, 13B. The reflecting surfaces 13A, 13B also areconstructed by forming 45-degree oblique surfaces on the distal ends ofthe optical guides 62A, 62B and either affixing mirrors to these obliquesurfaces or vapor-depositing aluminum or the like on these surfaces.

The end faces of the optical guides 62A, 62B on the side of thelight-projecting and photoreceptor elements are formed into curvedsurfaces 62 a, 62 b so as to project outwardly. The curved surfaces 62a, 62 b possess a lens function. Projected light from thelight-projecting element is collimated by this lens functioning portionand impinges upon the optical guide 62A. Light that is guided by theoptical guide 62B is condensed by its lens functioning portion andimpinges upon the photoreceptor element.

The distal ends of the optical guides 62A, 62B on the sides in back ofthe reflecting surfaces 13A, 13B are provided with reflecting members(only one of which is indicated at 64B in FIG. 15). These reflectingmembers are formed to have the reflecting surfaces 14A, 14B (not shown).The reflecting surfaces 14A, 14B of the reflecting members are formed byaffixing mirrors or by vapor-depositing aluminum, or they may beimplemented by prisms.

The sensing arm of the second modification also possesses the advantageof high strength and, in comparison with the sensing arm of the firstmodification, is easy to fabricate because it need not be separated intoa base and cover.

FIG. 16 illustrates another sensing principle. Here projected light ispassed through the object OB (the sensing area) three times. Projectedlight from the light-projecting element 21 of a preceding stage isreceived by the photoreceptor element 22 of a succeeding stage.

Light emitted by the light-projecting element 21 is reflected by thereflecting surface 13A, directed toward the object OB and passes throughthe object OB. This light is reflected by the reflecting surfaces 14AA,14BB, is directed toward the object OB and passes through the object OBagain. This light is further reflected by reflecting surfaces 13C, 13D,passes through the object OB a third time, is reflected by a reflectingsurface 13BB and is directed toward the photoreceptor element 22.

Since the object OB is traversed three times, the light received by thephotoreceptor element is attenuated greatly even if the object OB istransparent, thus making it possible to sense a transparent body muchmore accurately. If the transmittance of a transparent wafer is 92% andthe amount of received light in the absence of the wafer is 100, theamount of received light when the transparent wafer is present will be100×0.92×0.92×0.92=77.9. By setting a threshold value in the vicinityof, e.g., 86, a transparent wafer can be sensed. The S/N ratio of theamount of received light is larger in comparison with the firstembodiment, thus making it possible to sense a transparent wafer morestably.

It will suffice to provide the sensing arm with the light-projectingelement 21, a photoreceptor element 22A for receiving the light of apreceding stage, the reflecting surfaces 13A, 13C, 13D, and thereflecting surfaces 14A, 14B, 13B (which correspond to the reflectingsurfaces 14AA, 14BB, 13BB) for light of the preceding stage.

FIG. 17 illustrates an arrangement in which projected light passesthrough the object OB (the sensing area) four times. In comparison withthe first embodiment, the central reflecting surfaces 13C, 13D, 14CC,14DD have been added.

Projected light from the light-projecting element 21 is reflected by thereflecting surface 13A to pass through the object OB once, is reflectedby the reflecting surfaces 14AA, 14CC to pass through the object OB asecond time, is reflected by the reflecting surfaces 13CC, 13D to passthrough the object OB a third time, is reflected by the reflectingsurfaces 14DD, 14BB to pass through the object OB a fourth time, and isfinally reflected by the reflecting surface 13B to impinge upon thephotoreceptor element 22.

If the transmittance of a transparent wafer is 92% and the amount ofreceived light in the absence of the wafer is 100, the amount ofreceived light will be 100×0.92×0.92×0.92×0.92=71.6. This provides afurther improvement in S/N ratio. A transparent wafer can be sensed witheven greater stability.

It will suffice to provide the sensing arm with the light-projectingelement 21, the photoreceptor element 22, the reflecting surfaces 13A,13C, 13D, 13B and the reflecting surfaces 14A, 14C, 14D, 14B (whichcorrespond to the reflecting surfaces 14AA, 14CC, 14DD, 14BB) for thelight of the preceding stage.

The number of times light is passed through a transparent wafer (sensingarea) can be made five or more. The S/N ratio is improved even furtherand the sensing of transparent wafers is stabilized even further.

Second Embodiment

FIG. 18 illustrates part of a wafer sensor according to a secondembodiment. Two adjacent sensing arms are shown. Sensing arms the numberof which is one greater than the number of wafers to be sensed at onestroke are arrayed in a case (not shown) unidirectionally in one row.The wafers serving as the objects to be sensed are flat, plate-shapedobjects and are situated between sensing arms in such a manner that thedirection in which the sensing arms are arrayed and the surfaces of thewafers will be perpendicular.

The two adjacent sensing arms are indicated by characters 12 m and 12 n.Because these sensing arms are identical, first one of the sensing arms,namely 12 m, will be described.

The sensing arm 12 m has an arm frame 15 formed as an integral part ofthe mounting piece 16. A light-projecting element 21 m and aphotoreceptor element 22 m are accommodated within the mounting piece16. The light-projecting and photoreceptor elements 21 m, 22 m are drawnas solid-line and dashed-line circles, respectively, in FIG. 18 for thesake of convenience.

The connecting member at the distal end of the arm frame 15 m of sensingarm 12 m has an inner side formed to include two reflecting surfaces72Am, 72Bm as by affixing mirrors or vapor-depositing aluminum. Thereflecting surface 72Am reflects projected light, which is emitted fromthe light-projecting element 21 m and advances along one side of theframe 15 m, approximately perpendicular to the direction in which theprojected light advances and obliquely with respect to the direction(indicated by the dot-and-dash line M) in which the sensing arms arearrayed. The reflecting surface 72Bn reflects the light, which hasadvanced obliquely with respect to the array direction M, approximatelyperpendicularly downward so as to direct the light toward thephotoreceptor element 22 m. The photoreceptor surface of thephotoreceptor element 22 m is formed to have a slit 71 for limiting thelight that impinges upon the photoreceptor element 22 m.

Components of the sensing arm 12 n that are identical with those of thesensing arm 12 m are designated by like reference numerals with thecharacter “n” replacing the character “m”.

Light emitted from the light-projecting element 21 m of the sensing arm12 m is reflected by the reflecting surface 72Am, directed toward thereflecting surface 72Bn of the neighboring sensing arm 12 n, reflectedby the reflecting surface 72Bn and received by the photoreceptor element22 n via the slit 71.

The sensing principle is illustrated in FIGS. 19a and 19 b.

In a case where an opaque wafer 9A is present between the two sensingarms 12 m and 12 n in FIG. 19a, projected light from a reflectingsurface 72Am is reflected by the surface of the wafer 9A and does notreach a reflecting surface 72Bn. Naturally, the light does not reach thephotoreceptor element 22 n either.

Assume that a transparent or semi-transparent wafer 9B is presentbetween the two sensing arms 12 m and 12 n in FIG. 19b. The surface ofthe wafer 9B is perpendicular to the array direction M, and projectedlight from the reflecting surface 72Am impinges on the wafer 9Bobliquely.

Light which impinges obliquely upon the transparent or semi-transparentwafer 9B and passes through the wafer 9B sustains loss in terms of theamount of light owing to reflection (loss due to surface reflection,which depends upon the angle of incidence, and loss of light due toFresnel reflection internally of the wafer). Light that has passedthrough the wafer 9B is reflected by the reflecting surface 72Bn and isreceived by the photoreceptor element 22 n.

The amount of light received by the photoreceptor element 22 n is fairlysmall, in comparison with a case where light is received upon impingingon the wafer 9B perpendicularly and then passing through the wafer,owing to the above-mentioned loss in light quantity due to reflection oflight incident obliquely, and further the amount of loss is very greatas compared with a case where the wafer 9B is absent. Accordingly, aconsiderable difference develops in the amount of received light betweena case where a wafer is absent and a transparent or semi-transparentwafer is present, and it is possible to distinguish between these casesby using the levels of the photoreception signals. It will suffice toset a threshold value between the amount of light received in theabsence of a wafer and the amount of light received in the presence of atransparent or semi-transparent wafer.

When light passes through the transparent or semi-transparent wafer 9Bobliquely, the optic axis is displaced, as indicated by the broken lineand dot-and-dash line in FIG. 19b. As a result, the position at whichlight reflected by the reflecting surface 72Bn impinges upon thephotoreceptor element 22 n also is displaced. The slit 71 is formed atthe position at which the center of the light rays impinges in theabsence of a wafer. In the case where the wafer 9B is present, thecenter of the light rays incident upon the photoreceptor element 22 nwill be outside the slit 71 or, if inside the slit, on the edge thereof.The amount of light actually incident upon the photoreceptor element 22n, therefore, decreases considerably. Owing to provision of the slit 71,the amount of light received by the photoreceptor element in a casewhere a transparent or semi-transparent wafer is present decreasesconsiderably in comparison with a case where no wafer is present, thusmaking it possible to distinguish between these cases accurately. It ispreferred that the width and position of the slit 71 be decided, takinginto consideration such factors as the beam diameter and incidentposition of the incident light, in such a manner that the amount oflight incident upon the photoreceptor element 22 n will decrease as muchas possible if the wafer 9B is present.

It will suffice to drive the light-projecting elements and photoreceptorelements as follows in a case where a multiplicity of sensing arms arearrayed:

By opening the gate, which controls the passage of a photoreceptionsignal from the nth photoreceptor element 22 n (arrayed alongside themth light-projecting element) (n=m+1), at the timing at which the mthlight-projecting element 21 m is driven to emit projected light (orshortly after this timing), the output signal of this photoreceptorelement is accepted. The light-projecting element that is driven and thephotoreceptor element whose gate is opened are staggered one at a timeat a fixed period. That is, it will suffice to treat thelight-projecting element of one of the neighboring sensing arms and thephotoreceptor element of the other as one pair.

An arrangement may be adopted in which a light-projecting element isplaced at the position of the reflecting surface 72Am, a photoreceptorelement is placed at the position of the reflecting surface 72Bn, lightfrom the light-projecting element is made to impinge obliquely upon awafer (the object to be sensed) and light that has passed through thewafer obliquely is received by the photoreceptor element.

Since the optic axis of the light that has passed through the wafer (theobject to be sensed) obliquely is displaced, as described above, thepresence of a transparent or semi-transparent wafer can be sensed alsoby detecting this shift in the optic axis (the shift in the position ofincidence). The presence of a transparent or semi-transparent wafer maybe sensed based upon both a shift in the position of incidence and adecrease in amount of received light.

In the cases described above, a position sensing device (PSD) may beemployed instead of a photoreceptor element. FIG. 20 illustrates acircuit for sensing a transparent (or semi-transparent) wafer based uponposition of incidence and amount of received light.

It is assumed that 26 sensing arms 12 (indicated collectively atreference numeral 12) have been arrayed. The first sensing arm isprovided with only a light-emitting element 21 (indicated by LED1); noPSD is provided. A light-emitting element and PSD provided on the secondsensing arm are denoted by LED2 and PSD2, respectively. Similarly, thelight-emitting element and PSD of an ith (i=3-25) sensing arm aredenoted by LEDi and PSDi, respectively. The 26th sensing arm is providedwith only a PSD, which is denoted by PSD26. In a case where a wafer isnot present between the ith (i=1-25) sensing arm and a jth (j=i+1)sensing arm, or in a case where a transparent (or semi-transparent)wafer is present between them, the projected light from thelight-emitting element LEDi is received by PSDj (channel i).

A driver 102 is provided to drive the LEDs. The output of the driver 102is applied to any one of the LEDs via a projected-light gate switchingcircuit 103. The PSDs are one-dimensional PSDs whose two outputs I1, I2are amplified by respective ones of first stage amplifiers (25×2=50amplifiers are designated by reference numeral 104) and then applied toa photoreception gate switching circuit 105. The photoreceptor gateswitching circuit 105 allows the passage of an output from any one PSD.

The projected-light gate switching circuit 103 and photoreception gateswitching circuit 105 are controlled by a timing control circuit 101.The timing control circuit 101 controls the projected-light gateswitching circuit 103 in such a manner that the LEDs 1-25 are drivensequentially at regular time intervals channel by channel. The timingcontrol circuit 101 further controls the photoreception gate switchingcircuit 105 in such a manner that the output of the PSDj is allowed topass at the moment the LEDi is driven (at a timing somewhat later thanthis, strictly speaking). The timing control circuit 101 is providedwith the above-mentioned regular time intervals and other set inputsfrom an external input circuit 100.

The two outputs I1, I2 of the photoreception gate switching circuit 105are amplified by amplifiers 106, 107 and converted to voltage signalsV1, V2, respectively. The voltage signals V1, V2 are added by an addingcircuit 108. Further, V1/(V1+V2) is calculated by a dividing circuit109. The output of the dividing circuit 109 represents the position ofincident light on the PSD.

A threshold value for position and a threshold value for amount of lighthave been set in a memory 119 as digital data. This digital data is setfor each of the 25 PSDs (i.e., channel by channel). As a result, optimumthreshold values commensurate with variance in the characteristics ofthe PSDs are assured. When the LED of a corresponding channel is driven,the threshold value of this PSD is read out of the memory 119 andapplied to an arithmetic register 118. The threshold value for positionis converted to an analog voltage signal by a D/A converter 112 and thenapplied to a comparator 111 as a reference voltage. The threshold valuefor amount of light is converted to an analog voltage signal by a D/Aconverter 122 and then applied to a comparator 121 as a referencevoltage.

In the absence of a wafer between two adjacent sensing arms, thesensed-position output of the PSD of the sensing arm (the output of thedividing circuit 109) will take on a value corresponding to themidpoint. In a case where a transparent or semi-transparent wafer ispresent, the sensed-position output of the PSD will be higher or lowerthan the midpoint value. Assume here that the output will be lower thanthe midpoint value. It is assumed that the threshold value for positionhas been set between the level of the sensed-position output of the PSDin the absence of a wafer and the level of the sensed-position output ofthe PSD in the presence of a transparent or semi-transparent wafer. Theoutput of the dividing circuit 109 is applied to a comparator 111. Thelatter generates an output having the L level in the absence of a waferand an output having the H level in the presence of a transparent orsemi-transparent wafer.

The output of the dividing circuit 109 is applied also to a binarizingcircuit 114. The latter generates an H-level signal in a case where thesensed-position output of the PSD possesses a certain level other thanthat of a burn-out state (a state in which light does not impinge uponthe PSD owing to the presence of an opaque wafer, as a result of whichthe sensed-position output of the PSD takes on a very small value). TheH-level signal is applied to a switching circuit 113. When the output ofthe comparator 111 is at the H level, the switching circuit 113 turns onand its output also attains the H level. When the output of thecomparator 111 is at the L level, the output of the switching circuit113 is held at the L level.

In the absence of a wafer between two adjacent sensing arms, the output,which represents the amount of received light, of the PSD of the sensingarm (the output of the adding circuit 108) takes on the maximum value.If a transparent or semi-transparent wafer is present, the PSD outputrepresenting the amount of received light falls below the maximum value.It is assumed that the threshold value for amount of light has been setbetween the level of the PSD output representing the amount of receivedlight in the absence of a wafer and the level of the PSD outputrepresenting the amount of received light in the presence of atransparent or semi-transparent wafer. The output of the adding circuit108 is applied to a comparator 121. The latter generates an outputhaving the L level in the absence of a wafer and an output having the Hlevel in the presence of a transparent or semi-transparent wafer.

The output of the adding circuit 108 is applied also to a binarizingcircuit 124. The latter generates an H-level signal in a case where thePSD output representing the amount of received light possesses a certainlevel other than almost zero (a state in which light does not impingeupon the PSD owing to the presence of an opaque wafer, as a result ofwhich the sensed-position output of the PSD takes on a very smallvalue). The H-level signal is applied to a switching circuit 123. Whenthe output of the comparator 121 is at the H level, the switchingcircuit 123 turns on and its output also attains the H level. When theoutput of the comparator 121 is at the L level, the output of theswitching circuit 123 is held at the L level.

When a transparent (or semi-transparent) wafer is present, the outputsof the switching circuits 113, 123 both are at the H level. The AND ofthese H-level signals is taken by an AND gate 125, and the result of theAND operation is output via an output circuit 126.

By thus taking the AND between the sensed-position output and the outputrepresenting the amount of received light, the reliability with whichthe presence of a transparent (or semi-transparent) wafer is sensed isimproved.

It goes without saying that the function of the circuitry enclosed bythe dot-and-dash line 110 can be implemented not only by hardware butalso by a programmed CPU or the like.

An arrangement may be adopted in which the binarizing circuits 114, 124and the switching circuits 113, 123 are eliminated and the outputs ofthe comparators 111, 121 are applied to the AND gate 125 directly.

By setting two different threshold values with respect to thesensed-position output of the PSD, it is possible to discriminate (a)absence of a wafer, (b) a transparent (or semi-transparent) wafer, and(c) an opaque wafer. By setting two different threshold values withrespect to the PSD output representing the amount of received light, itis possible to discriminate (d) absence of a wafer, (e) a transparentwafer, and (f) a semi-transparent (or opaque) wafer. Absence of a waferis determined by taking the AND between (a) and (d), presence of atransparent wafer is determined by taking the AND between (b) and (e),presence of a semi-transparent wafer is determined by taking the ANDbetween (b) and (f), and presence of an opaque wafer is determined bytaking the AND between (c) and (f). Thus, various states can be sensedwith high reliability. Three different threshold values may be set withrespect to the PSD output representing the amount of received light, thepresence of an opaque wafer may be discriminated and this may beutilized to determine the presence of an opaque wafer.

Third Embodiment

A third embodiment relates to a wafer sensor in which the number oflight-projecting elements and photoreceptor elements is reduced to halfthat of the first and second embodiments. The third embodiment has twomodes depending upon the direction of the projected light.

The first mode will be described with reference to FIGS. 21 to 24.

In particular, as shown in FIGS. 21 and 22, a multiplicity of sensingarms 12Dm, 12Lm, 12Dn, etc., are arrayed in a single row. The connectingmembers at the distal ends of the arm frames 15 of these sensing armshave inner sides formed to include reflecting surfaces 82Dm, 821 m,82Dn, respectively, each comprising two oblique surfaces 82A, 82B havingan angle of 45° with respect to the longitudinal direction of the arm.The two oblique surfaces 82A, 82B of these reflecting surfaces are alsoreflecting surfaces and are formed as by affixing mirrors orvapor-depositing aluminum.

A light-projecting element 21Lm (only the reference character and notthe element is shown) is accommodated within the mounting piece 16 ofthe sensing arm 82Lm, and the projected light thereof is directed towardthe reflecting surface 82Lm via a lens 83. Photoreceptor elements 22Dm,22Dn (only the reference characters and not the elements are shown) areaccommodated within the mounting pieces 16 of the sensing arms 82Dm,82Dn neighboring on both sides of the sensing arm 82Lm. Light projectedfrom the light-projecting element 21Lm is split into two portions by thereflecting surface 82Lm, and these portions are directed towardrespective ones of the reflecting surfaces 82Dm (82B), 82Dn (82A) of thesensing arms 12Dm, 12Dn, respectively, neighboring on both sides,reflected by these reflecting surfaces and received by the correspondingphotoreceptor elements 22Dm, 22Dn.

In order to form sensing areas of 25 channels, the case 11 is providedwith 26 of the sensing arms as shown in FIG. 23. Among these sensingarms, those indicated by reference characters 12L1, 12L2, . . . , 12L13incorporate light-projecting elements and those indicated by referencecharacters 12D1, 12D2, . . , 12D13 incorporate photoreceptor elements.Thus, the sensing arms which include the light-projecting elements andthe sensing arms which include the photoreceptor elements are arrangedalternatingly. Since one sensing arm is provided with only alight-projecting element or a photoreceptor element, the number oflight-projecting and photoreceptor elements can be reduced. The case 11is provided with a connector 42.

The sensing areas of the respective channels are sensed sequentially atregular time intervals. Driving of the light-projecting elements andgating control of the photoreception signals of the photoreceptorelements will be described later.

With reference to FIG. 24, projected light (SP indicates the projectedbeam from the light-projecting element) from the reflecting surface 82Lmof the sensing arm 12Lm incorporating the light-projecting elementadvances in the direction in which the sensing arms are arrayed. If theopaque wafer 9 is present between the two sensing arms 12Lm and 12Dm (or12Lm and 12Dn), the projected light will be blocked and will not bereceived by the photoreceptor element of the neighboring sensing arm12Dm or 12Dn. Thus it is possible to sense an opaque wafer. If athreshold value is chosen wisely, a transparent (or semi-transparent)wafer can be sensed as well.

The second mode will now be described with reference to FIGS. 25 and 26.This mode is identical with the first mode in that the sensing arm 12Lmincorporates the light-projecting element 21Lm, the sensing arms 12Dm,12Dn incorporate the photoreceptor elements 22Dm, 22Dn, and the sensingarms are arrayed in the case 11 in a single row, as illustrated in FIG.23. This mode differs from the first mode in that the arrangement issuch that projected light advances through the sensing areas obliquelywith respect to the direction in which the sensing arms are arrayed, ina manner the same as that of the second embodiment shown in FIG. 18.

The sensing arm 12Lm is formed to have reflecting surfaces 82Lm 1, 82Lm2 for splitting the projected light from the light-projecting element12Lm into two portions and reflecting these obliquely in oppositedirections, and the sensing arm 12Dm (or 12Dn) is formed to havereflecting surfaces 82Dm 1, 82Dm 2 (or 82Dn 1, 82Dn 2) for guidinglight, which has advanced obliquely, to the photoreceptor element 22Dm(or 22Dn).

As shown in FIG. 26, projected light impinges obliquely upon the surfaceof the wafer 9, as described in conjunction with the second embodiment.Loss in terms of the amount of light increases and it is possible tosense not only an opaque wafer but also a transparent orsemi-transparent wafer.

FIG. 27 illustrates an example of a sensing circuit applied to wafersensors of both the first and second modes described above.

This sensing circuit includes a CPU 130. The CPU 130 is accompanied by aROM 139 and other memories, and an oscillating circuit 138 for applyinga clock signal to the CPU 130 is connected to the CPU. An input circuitand an output circuit (neither of which are shown) are connected to theCPU 130.

In order to sense 25 channels (25 ch), 26 sensing arms are provided. Asmentioned above, each sensing arm is provided with either alight-projecting element or a photoreceptor element. Theselight-projecting elements are represented by L1, L2, L3, . . . , L13,and the photoreceptor elements are represented by D1, D2, D3, . . . ,D13. The light-projecting elements and photoreceptor elements arearrayed in the following order: L1, D1, L2, D2, L3, D3, . . . , L13,D13.

A driving circuit 132 generates drive pulses for light-projectingelements in response to a light-emission timing pulse PLS provided bythe CPU 130. A projected-light gate 131 decodes light-projection controlsignals CTL1-CTL3 provided by the CPU 130 to open any one gate of 13light-projecting elements L1-L13 of a light-projecting unit 133 andapply the drive pulse to one light-projecting element only.

A photoreceptor unit 134 includes 13 photoreceptor elements D1-D13. Thephotoreception signals of these photoreceptor elements are applied to aphotoreceptor gate 135. The photoreceptor gate 135 decodesphotoreception control signals P1-P3 provided by the CPU 130 to open agate in such a manner that any one photoreception signal only will beapplied to a photoreception amplifier 136.

The output of the photoreception amplifier 136 is held by a peak-holdcircuit 137. While thus being held, the photoreception signal isaccepted at an A/D port of the CPU 130. The peak-hold circuit 137 isreset at the timing at which the light-projecting elements start to bedriven.

Indicator lamps 140 display the result of object detection for eachchannel of the 25 channels.

FIG. 28 illustrates the manner in which the 13 light-projecting elementsL1-L13 and 13 photoreceptor elements D1-D13 are driven, or the manner inwhich the gates are controlled, and the manner in which an object issensed.

The object sensing operation is carried out at regular intervalssuccessively in the manner of the first channel, second channel, . . . ,25th channel. On the first channel, the light-projecting element L1 isdriven and the gate of photoreceptor element D1 opens. On the secondchannel, the light-projecting element L2 is driven and the gate ofphotoreceptor element D1 opens. On the third channel, thelight-projecting element L2 is driven and the gate of photoreceptorelement D2 opens. Thus, with the exception of the light-projectingelement on the leading end, light-projecting elements are driven for alength of time equivalent to two consecutive channels. Gates of thephotoreceptor elements, with the exception of the photoreceptor elementon the trailing end, are opened twice over a period of time equivalentto two channels and pass photoreception signals that are based uponprojected light from two different light-projecting elements.

Two threshold values TH1, TH2 have been set and are stored in the ROM139. The threshold value TH1 is for discriminating a photoreceptionlevel indicative of the absence of a wafer and the photoreception levelindicative of a transparent (or semi-transparent) wafer, and thethreshold value TH2 is for discriminating the photoreception levelindicative of a transparent (or semi-transparent) wafer and of an opaquewafer.

When absence of a wafer has been discriminated in either discriminationbased upon threshold value TH1 or discrimination based upon thresholdvalue TH2, the final decision is that a wafer is absent. In a case wherediscrimination using the threshold value TH1 indicates presence of awafer and discrimination using the threshold value TH2 indicates absenceof a wafer, the decision rendered is that a transparent (orsemi-transparent) wafer is present. In a case where presence of a waferis indicated by discrimination using both of the threshold values TH1and TH2, the decision rendered is that an opaque wafer is present.

In order to improve reliability, the above-described measurements may berepeated a plurality of times and only if the same results are obtainedin all measurements would the results be output as the final detectionoutput.

This makes it possible to sense, at one stroke, at which of amultiplicity of positions wafers are present (or absent) as well as thekinds of wafers. Moreover, it is possible to distinguish betweentransparent and opaque wafers. The number of light-projecting elementsand the number of photoreceptor elements are approximately the half ofthe number of channels.

Fourth Embodiment

A fourth embodiment relates to the structure of a static-electricityshield of a wafer sensor. The structure internally of the case of thewafer sensor according to the first embodiment will be described withreference to FIGS. 29 to 33.

A multiplicity of the sensing arms 12 are provided on one side of thewafer case 11, as mentioned above, and the arm frames thereof projectoutwardly of the case. The wafer case 11 accommodates a printed circuitboard 152 on which a sensing circuit is mounted. The leads of thelight-projecting elements 21 and photoreceptor elements 22 of thesensing arms 12 are connected by soldering to a wiring pattern formed onthe circuit board 152. One side of the wafer case 11 is open and iscovered by a freely removably cover 151.

As mentioned above, the photoreceptor elements 22 are accommodated inthe shield case 23 (see FIG. 4). The shield case 23 is connected to theground of printed circuit board 152.

A shield plate 153 is provided so as to cover the photoreceptor elements22 and the entire surface of the printed circuit board 152. The shieldplate 153 is formed from a thin, electrically conductive material andthe periphery thereof is bent to thereby cover not only one side of theboard 152 but also the periphery thereof. The shield plate 153 isconnected to a ground wiring pattern (ground line) of the board 152.

Shield pieces 155 extend at right angles from the front edge of theshield plate 153 at positions between the sensing arms 12. The shieldpieces 155 are inserted into holes 156 formed in the case 11. The spacesbetween the sensing arms 12 are covered and shielded by the shieldpieces 155. Holes provided for shrinkage cavatiy prevention when theresin of the case is molded may be utilized advantageously as the holes156.

The input/output cable 41 is led out from the board 152 and a connector157 is connected to the distal end of the cable 41.

The underside of the board 152 is provided with a sensing circuit thenecessary portions of which are covered by an auxiliary shield member158. The auxiliary shield member 158 also is formed by bending the edgeof a thin, plate-shaped body consisting of an electrically conductivematerial, and the leads thereof are connected to a ground line on theboard 152 by soldering.

Even if the wafers 9 become charged with static electricity, whatpenetrates the spaces between the wafers 9 are the distal ends of thesensing arms 12, and there is an insulating distance between the wafers9 and the light-projecting elements 21 and photoreceptor elements 22, asmentioned earlier. The static electricity that has charged the wafers 9,therefore, can be prevented from discharging into the light-projectingelements 21 or photoreceptor elements 22 and sensing circuit, or suchdischarge is made extremely difficult to occur.

Even if the static electricity that has charged the wafers 9 shouldhappen to be discharged from the wafers, the photoreceptor elements 22and sensing circuit, etc., are shielded by the grounded shield plate153, shield case 23, shield piece 155 and auxiliary shield member 158.As a result, malfunction and destruction of the sensing circuit,especially the photoreceptor elements 22, due to electric discharge canbe prevented. The shield piece 155 is effective in positively shieldingagainst electric discharge from the edges of the wafers to the frontside of the case.

The entirety of the printed circuit board 152 may be covered withshielding foil. Conversely, only the shield case 23 of the photoreceptorelements 22 may be provided. The shield piece 155 may be deleted.

What is claimed is:
 1. A multiple transmission-type photoelectric sensorin which a plurality of sensing arms are provided in spaced-apartrelation on a sensor case so as to extend outwardly of the case,comprising: a plurality of light-projecting elements and a plurality ofphotoreceptor elements are provided inside the sensor case; one pair ofthe light-projecting and photoreceptor elements correspond to eachsensing arm; and a distal end of each sensing arm is provided with afirst deflecting member for directing projected light from thecorresponding light-projecting element toward a neighboring sensing armon one side, a second deflecting member for directing projected lightfrom the neighboring sensing arm on one side toward the correspondingphotoreceptor element, and a third deflecting member for returning theprojected light from a neighboring sensing arm on the other side to theneighboring sensing arm on the other side.
 2. A multipletransmission-type photoelectric sensor according to claim 1, whereinthere are provided at least one of first light guides for guidingprojected light from the light-projecting elements to the firstdeflecting members and second light guides for guiding light from thesecond deflecting members to the photoreceptor elements.
 3. A multipletransmission-type photoelectric sensor according to claim 1, having:drive means for driving a plurality of light-projecting elementssequentially at predetermined time intervals; and means for capturing,in sync with driving of the light-projecting elements, a photoreceptionsignal of a prescribed photoreceptor element which receives projectedlight from a light-projecting element that is driven.
 4. A multipletransmission-type photoelectric sensor according to claim 3, furtherhaving decision means having at least one threshold value fordiscriminating an output signal of a photoreceptor element based uponthis threshold value, thereby outputting a detection signal indicativeof an object to be sensed.
 5. A multiple transmission-type photoelectricsensor according to claim 4, wherein said threshold value is fordistinguishing between absence of a transparent body and presence of atransparent body.
 6. A multiple transmission-type photoelectric sensoraccording to claim 4, wherein said threshold value is for distinguishingbetween absence of a semi-transparent body and presence of asemi-transparent body.
 7. A multiple transmission-type photoelectricsensor according to claim 4, wherein said threshold value is fordistinguishing between a transparent body and a semi-transparent body.8. A multiple transmission-type photoelectric sensor according to claim4, wherein said threshold value is for distinguishing between atransparent body and an opaque body.
 9. A multiple transmission-typephotoelectric sensor according to claim 4, wherein said threshold valueis for distinguishing between a semi-transparent body and an opaquebody.
 10. A multiple transmission-type photoelectric sensor according toclaim 4, wherein said threshold value is for distinguishing betweenabsence or presence of an object.
 11. A multiple transmission-typephotoelectric sensor in which a plurality of sensing arms are providedin spaced-apart relation on a sensor case so as to extend outwardly ofthe case, comprising: a plurality of light-projecting elements and aplurality of photoreceptor elements are provided inside the sensor case;one light-projecting element or one photoreceptor element corresponds toeach sensing arm; the distal end of the sensing arm that corresponds tothe light-projecting element is provided with a first deflecting memberwhich splits the projected light from the light-projecting element intotwo portions and directs these two portions toward the neighboringsensing arms arranged respectively on either side of the sensing arm;and the distal end of the sensing arm that corresponds to thephotoreceptor element is provided with a second deflecting member whichdirects the projected light from the neighboring sensing arm toward thecorresponding photoreceptor element, wherein the first deflecting memberdirects the optical path of the projected light obliquely with respectto a direction in which the sensing arm and the neighboring sensing armsare arrayed, and the second deflecting member directs the light, whichhas advanced obliquely with respect to the direction in which thesensing arm and the neighboring sensing arms are arrayed, toward thephotoreceptor element, wherein a front side of the photoreceptor elementis provided with a slit for limiting impinging light, and wherein thephotoreceptor element comprises a position sensing device that generatesa sensed position output signal which changes based on a position wherethe light impinges.
 12. A multiple transmission-type photoelectricsensor according to claim 11, further having decision means fordiscriminating the sensed-position output signal of the position sensingdevice and outputting a signal indicative of sensing of an object to besensed.
 13. A multiple transmission-type photoelectric sensor accordingto claim 11, further having: first discriminating means fordiscriminating the sensed-position output signal of the position sensingdevice; second discriminating means for discriminating an output signalof the position sensing device indicative of amount of received light;and decision means for subjecting a discrimination output of the firstdiscriminating means and a discrimination output of the seconddiscriminating means to a logic operation and outputting a signalrelating to sensing of an object to be sensed.
 14. A transmission-typephotoelectric sensor which includes a first sensing arm and a secondsensing arm and a third sensing arm disposed respectively on either sideof the first sensing arm across sensing areas defined therebetween,comprising: the first sensing arm having a first splitting anddeflecting member at a distal end thereof for splitting, into twoportions, projected light received from a light-projecting element,which projected light is introduced from a base end of the first sensingarm and advances along the longitudinal direction of the arm, the firstsplitting and deflecting member deflecting these split portions of lightapproximately at right angles, and directing these split portions oflight toward the sensing areas on both sides; the second and thirdsensing arms respectively having second and third deflecting members atrespective distal ends thereof for deflecting light, which has advancedfrom the first splitting and deflecting member through the sensingareas, approximately at right angles, causing the light to advance alongthe longitudinal direction of the arms and introducing the light tophotoreceptor elements, wherein said first splitting and deflectingmember deflects projected light in an oblique direction with respect toa direction in which the first, second and third sensing arms arearrayed, wherein said second and third deflecting members deflect light,which has advanced through the sensing areas obliquely, in thelongitudinal direction of the second and third sensing arms,respectively, wherein the photoreceptor element comprises a positionsensing device that generates a sensed position output signal whichchanges based on a position where the light impinges.
 15. Atransmission-type photoelectric sensor in which at least a first sensingarm and a second sensing arm are disposed so as to oppose each otheracross a sensing area defined therebetween, comprising: the firstsensing arm having a first deflecting member at a distal end thereof fordeflecting received projected light from a light-projecting element,which projected light is introduced from a base end of the first sensingarm and advances along the longitudinal direction of the first sensingarm, the first deflecting member deflecting the received projected lightin a direction that is approximately perpendicular to the longitudinaldirection of the arm and oblique with the respect to a direction inwhich the first and second sensing arms are arrayed, thereby directingthe light toward the sensing area; and the second sensing arm having asecond deflecting member at a distal end thereof for deflecting light,which has advanced from the first deflecting member of the first sensingarm obliquely through the sensing area, in an approximatelyperpendicular direction, causing the light to advance along thelongitudinal direction of the second sensing arm and introducing thelight to a photoreceptor element, wherein the photoreceptor element alsocomprises a position sensing device that generates a sensed positionoutput signal that changes based on a position where the light impinges.16. A transmission-type photoelectric sensor according to claim 15,wherein both the first sensing arm and the second sensing arm areprovided with the first and second deflecting members.
 17. Atransmission-type photoelectric sensor according to claim 15, whereinthe front side of the photoreceptor element is provided with a slit forlimiting impinging light.
 18. A transmission-type photoelectric sensoraccording to claim 15, further comprising: first discriminating meansfor discriminating a level of the sensed-position output signal from theposition sensing device; second discriminating means for discriminatingthe level of a signal from the position sensing device indicative ofsensed amount of received light; and means for subjecting results ofdiscriminating by the first discriminating means and seconddiscriminating means to a logic operation.